The Waterloo-Pickney trail sounds like what your after. About 35 miles long, several campgrounds, rolling hills, many small lakes. Crooked Lake is near the east TH. Likely to be very wet right now with the snow just going out but with the ground still frozen, it's kind of wet and boggy any time of the year. I would buy or go to the library and get the Jim DuFresne book "50 Hikes in Michigan". A disadvantage, this trail is not a loop.
There is a 17 mile loop trail near me (Toledo OH area) in Oak Openings Metropark with camping. Also a 22 mile out and back along the Maumee River, part of the Buckeye Trail, also with camping possibilities. If either of these two sound good, PM me.
Dean